1. Field of the Invention
This invention relates to an optical scanning apparatus. This invention particularly relates to an optical scanning apparatus, which is provided with an optical waveguide type of electro-optic device comprising an optical waveguide and grating-shaped electrodes located on the optical waveguide such that an optical wave guided through the optical waveguide may be selectively diffracted in accordance with the condition, under which a voltage is applied across the grating-shaped electrodes, the guided optical wave being thereby modulated or the direction of the optical path of the guided optical wave being thereby changed over.
2. Description of the Prior Art
Optical scanning recording apparatuses have heretofore been used wherein a light beam, which serves as recording light, is modulated in accordance with an image signal, a recording material (such as a photosensitive material) is scanned with the modulated light beam in a main scanning direction and in a sub-scanning direction, and an image represented by the image signal is thereby recorded on the recording material. Also, optical scanning read-out apparatuses have heretofore been used wherein a recording material, on which an image has been recorded, is scanned with a light beam, which serves as reading light, in the main scanning direction and in the sub-scanning direction, light radiated out of the recording material during the scanning (i.e. light reflected by the recording material, light having passed through the recording material, or light emitted by the recording material) is detected, and the image recorded on the recording material is thereby read out.
In the aforesaid types of optical scanning apparatuses, it is often necessary that the light beam can be modulated or that the direction of the optical path of the light beam can be changed over. For such purposes, it has been proposed to utilize an optical waveguide type of electro-optic device as disclosed in, for example, Japanese Unexamined Patent Publication No. 2(1990)-931. The disclosed optical waveguide type of electro-optic device comprises an optical waveguide having electro-optic effects, grating-shaped electrodes (hereinafter referred to as the "EOG electrodes") located on the optical waveguide so as to form an electro-optic grating in the optical waveguide, and a driving circuit for applying a voltage across the EOG electrodes. An guided optical wave, which is guided through the optical waveguide, is thereby selectively diffracted in accordance with the condition, under which the voltage is applied across the EOG electrodes.
In cases where the optical waveguide type of electro-optic device is used, either one of a diffracted optical wave and an undiffracted optical wave (i.e. a zero-order optical wave) can be utilized as the scanning optical wave (i.e. the scanning light beam), and the scanning optical wave can be modulated in accordance with whether it is or is not diffracted or in accordance with the extent of diffraction. Also, an optical switch can be constructed which changes over the direction of the optical path of the guided optical wave in accordance with whether the guided optical wave is or is not diffracted.
Ordinarily, in the aforesaid optical waveguide type of electro-optic device, it is necessary that a buffer layer, which may be constituted of SiO.sub.2, or the like, is located between the EOG electrodes and the optical waveguide in order to eliminate scattering and absorption of the guided optical wave by the EOG electrodes.
However, it has been found that, in the optical waveguide type of electro-optic device provided with the buffer layer, the so-called "DC drift phenomenon" easily occurs, i.e. the applied voltage vs. diffraction efficiency characteristics easily fluctuate during the application of the voltage across the EOG electrodes.
If the optical waveguide type of electro-optic device is utilized in an optical scanning apparatus and the DC drift phenomenon occurs in the optical waveguide type of electro-optic device, the optical power of the scanning optical wave will fluctuate. Therefore, the image recording operation or the image read-out operation cannot be carried out accurately.